[issue17570] Improve devguide Windows instructions

[Steve Dower]

Steve Dower added the comment:

Right at the end of the patch, ".hg/hgrc" also exists on Windows and is not the same as mercurial.ini - hgrc here is per-repo and it seems from the context that this is relevant here.

Otherwise, the rest looks good to me. I'll merge it in when I get a chance, unless someone else gets there first.

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<http://bugs.python.org/issue17570>
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